1. Field of Invention
This invention relates generally to improved glass-to-titanium seals and, more particularly, to prenitriding titanium so as to avoid deleterious interfacial reactions that occur between standard sealing glasses and titanium Such interfacial reactions often prevent strong hermetic glass-to-titanium seals. Disclosed is a method for prenitriding titanium and making improved silicate glass-to-titanium seals appropriate for use in electronic components.
2. Description of the Related Art
In a field of electronic components design and utilization, titanium is a very useful material because it has outstanding strength-to-weight ratio, extremely good chemical durability in corrosive environments and moderately high temperature stability. Prior to the present invention, however, because no reliable glass-to-titanium sealing technology existed, titanium could not be used in devices that required truly durable and hermetic sealing. Commercial silicate glasses are typically recommended for sealing to titanium both because they have favorable physical and chemical properties and because they are readily available and relatively inexpensive. (See H. Rawson, et al., "The Glass Sealing Properties of Titanium and Zirconium", Br. J. Appl. Phys., 5 352 (1954), W. H. Kohl, Handbook of Materials and Techniques for Vacuum Devices, p. 401, Reinhold Publishing Corp, N.Y. (1967).) At sealing temperatures (above 900.degree. C.), however, silicate glasses rapidly react with the titanium metal to create bubbles due to alkali volatilization. Also an interfacial silicide phase forms which is deleterious to the hermeticity and strength of the glass-to-metal seal. For example, pin seals between silicate glasses and titanium have been shown to be only about half as strong as standard seals used in electronic components. It is believed that the lack of strength and hermeticity is due to poor adherence between the glass and the silicide. (A. Passerone, et al., "The Titanium-Molten Glass System: Interactions and Wetting", J. Mat. Sci., 12, 2465 (1977), Z. Feipeng, et al., "Wetting and Reactions in Glass/Titanium Systems", Proceedings of the 33rd Pacific Coast Regional Meeting of the American Ceramic Society, San Francisco, Calif., 76 (1980 ), R. K. Brow and R. D. Watkins, "Reactions and Bonding Between Glasses and Titanium", Proc. Winter Meeting ASME, MD-4, 25 (1987)).
Efforts to improve glass-to-titanium seals have primarily involved either altering sealing conditions for silicate glass or using different glass compositions which are less susceptible than silicate glass to forming interfacial. Layers in reaction with titanium. Early studies showed, for example, that shortened sealing times could help alleviate problems with differentials in thermal coefficients of expansion between the silicate glass and the titanium, while tending to decrease the rates of reaction at the interface between the glass and metal. (Rawson, et al.) The interfacial reactions, however, proved to be rapid, indeed, and high quality hermetic seals between titanium and silicate glasses still could not be obtained.
More recently, boroaluminate glasses have been demonstrated to form hermetically good seals in titanium components. (R. K. Brow and R. D. Watkins, "Sealing Glasses for Titanium and Titanium Alloys", U.S. Pat. No. 5,104,738, issued 1992). Although at sealing temperatures titanium still reacts initially with the boroaluminate glasses to form a thin interfacial boride layer, the reaction quickly subsides apparently because the interfacial boride passivates the titanium to prevent the types of uncontrolled deleterious reactions that prohibit formation of adequate seals between titanium and silicate glasses. While the mechanical characteristics of these seals are quite good, the boroaluminate glasses which are useful in a number of applications unfortunately cannot withstand many of the corrosive environments for which titanium components are needed.
There is an ongoing need for titanium components with hermetic glass-to-metal seals in a wide variety of government and commercial applications ranging from pyrotechnics and explosives to biomedical implant devices. In such applications, the high corrosion resistance of titanium can be extremely valuable, especially where chemically durable and widely available silicate glasses are used to seal components.